As the mobile network technologies have made an evolution and the service model has developed and transformed towards multimedia comprehensive applications including voice, video and data, the conventional microwave of plesiochronous digital hierarchy (PDH)/synchronous digital hierarchy (SDH) based on a fixed transfer bandwidth cannot well satisfy the requirements of mobile services on the bandwidth of the retransmission network. Thus, microwave equipment in a new generation that supports an adaptive modulation (AM) function has been introduced. The microwave equipment that supports the AM function can dynamically adjust an air interface transfer bandwidth. When an air interface link quality is quite high (for example, in a nice weather), the equipment may adopt a high modulation mode, for example, 128 quadrature amplitude modulation (128 QAM), so as to provide a large-bandwidth transfer capability. When the air interface link quality is deteriorated (for example, in a heavy rain or thick fog), the equipment may adopt a low modulation mode, for example, quadrature phase shift keying (QPSK), so as to enhance an anti-interference capability of the link, thereby enhancing the transmission quality of important user data.
The microwave radio frequency power amplifier circuit has a nonlinear distortion problem, and the degree of distortion increases as the increasing of the transmit power. The microwave system has a high requirement on a linearity of the radio frequency power amplifier circuit when adopting the high modulation mode, and has a low requirement on a linearity when adopting the low modulation mode. In order to reduce the nonlinear distortion of the power amplifier circuit, a manner of reducing the radio frequency transmit power may be adopted, which is referred to as power back-off. In the microwave system, a microwave system gain equals a sum of a transmit power and a receiving sensitivity value of the microwave system.
If an AM microwave system adopts a manner of a constant radio frequency transmit power of the conventional microwave, the transmit power needs to be backed off to a transmit power value of the highest modulation mode. If the system works at a lower modulation mode, a maximum system gain cannot be achieved, so that the anti-fading capability of the system is reduced.
Meanwhile, an automatic transmit power control (ATPC) technique has been introduced to the AM microwave system, and the ATPC technique sets an upper threshold value and a lower threshold value for a receive power of the microwave system. When a receive power is lower than the lower threshold value, a sending side is notified through a protocol to reduce the transmit power; and when the receive power is higher than the upper threshold value, the sending side is notified through a protocol to increase the transmit power. As the modulation mode of the sending side of the AM microwave system varies at any time, if the transmit power of the sending side is simply controlled by the receiving side, the transmit power of the sending side may possibly exceed the maximum transmit power allowed by the current modulation mode, thus causing a service impairment.